Dry ESP

Selection of these equipment should be based on the following considerations  

Volume of gases (mu /h) which will be passed through the ESP. 

Pressure at which the gases will enter the unit. 

Gas composition, especially concentration of corrosive gases like SO, HCl. Moisture content in gases (since this tends to form acid droplets). 

Size distribution of these particles. This data must be provided to the vendor, in the form of particle size (microns) and corresponding percentage (smaller the particle size, say 1-2 pm, it will be more and more difficult to separate them out from the gas stream. 

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Other Considerations In general, dry ESPs operate most efficiently with dust resistivities between 5x10 and 2 x lO ohm-cm. In general, the most diffieult particles to collect are those with aerodynamie diameters between 0.1 and 1.0 / m. Particles between 0.2 and 0.4 m usually show the most penetration. This is most likely a result of the transition region between field and diffusion charging. 

Electrodes in the center of the flow lane are maintained at high voltage and generate the electrical field that forces the particles to the collector walls. In dry ESPs, the collectors are knocked, or "rapped," by various mechanical means to dislodge the particulate, which slides downward into a hopper where they are collected. 

Other Considerations Dust resistivity is not a faetor for wet ESPs, beeause of the high humidity atmosphere whieh lowers the resistivity of most materials. Partiele size is mueh less of a faetor for wet ESPs, compared to dry ESPs. Mueh smaller particles ean be efficiently colleeted by wet ESPs due to the laek of resistivity concerns and the reduced reentrainment (Flynn, 1999). 

An ESP is a particulate control device that uses electrical forces to move particles entrained within an exhaust stream onto collection surfaces. The basic theory has already been described under dry ESPs, but a brief summary here is included, with... 

Unlike dry ESPs, resistivity of the collected material is generally not a major factor in performance. Because of the high humidity in a wet ESP, the resistivity of particles is lowered, eliminating the "back corona" condition. The frequent washing of the pipes also limits particle buildup on the collectors. 

Another advantage is that wet ESPs can collect sticky particles and mists, as well as highly resistive or explosive dusts. The continuous or intermittent washing with a liquid eliminates the reentrainment of particles due to rapping which dry ESPs are subject to. The humid atmosphere that results from the washing in a wet ESP enables them to collect high resistivity particles, absorb gases or cause pollutants to condense, and cools and conditions the gas stream. Liquid particles or aerosols... 

Wet ESPs are used in situations for which dry ESPs are not suited, such as when the material to be collected is wet, sticky, flammable, or explosive, or has a high resistivity. Therefore, producer gas streams with tar represent a problem. 

These features can make the wet ESP work better than the dry ESP. 

These features make the wet ESP a device better than the dry ESP, and one may find the higher initial cost worth it due to the improved performance. The life of downstream equipment improves due to better removal of erosive, corrosive particulate matter lay the wet ESP. 

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